Patients with OSA have recurrent apnoeas or hypopnoeas during sleep that are only terminated by the patient arousing. These recurrent events cause sleep fragmentation and stimulation of the sympathetic nervous system. This can have severe consequences for the patient including day-time sleepiness (with the attendant possibility of motor-vehicle accidents), poor mentation, memory problems, depression and hypertension. Patients with OSA are also likely to snore loudly, thus also disturbing their partner's sleep. The best form of treatment for patients with OSA is constant positive airway pressure (CPAP) applied by a blower (compressor) via a connecting hose and mask (patient interface). The positive pressure prevents collapse of the patient's airway during inspiration, thus preventing recurrent apnoeas or hypopnoeas and their sequelae.
Positive airway pressure may be delivered in many forms. For example, a positive pressure level may be maintained across the inspiratory and expiratory levels of the patient's breathing cycle at an approximately constant level. Alternatively, pressure levels may be adjusted to change synchronously with the patient's breathing cycle. For example, pressure may be set at one level during inspiration and another lower level during expiration for patient comfort. Such a pressure treatment system may be referred to as bi-level. Alternatively, the pressure levels may be continuously adjusted to smoothly replicate changes in the patient's breathing cycle. A lower pressure setting during expiration may generally be referred to as expiratory pressure relief.
In providing such changes to pressure and/or detecting conditions for making adjustments to the treatment pressure, it can be helpful to have a measure of patient respiratory flow. For example, a measure of patient respiratory flow may be utilized to detect when a patient changes from inspiration to expiration for determining when to deliver expiratory pressure treatment settings or inspiratory pressure treatment settings. Similarly, a measured patient respiratory flow signal may be utilized to detect patient flow limitation for purposes of making treatment pressure adjustments. Such adjustments are illustrated in the patent in U.S. Pat. No. 5,704,345. For these purposes, a measured flow signal may be derived from a flow sensor such as a differential pressure transducer or pnuemotachograph.
It may be desirable to develop further methods and devices for estimating flow to improve existing methods and devices and/or to develop new pressure treatment and detection methods and devices.